1. Field of the Invention
The present invention relates to an optoelectronic coupler for optical fibers in which possibilities are provided for extracting an adjustable fraction of the light energy carried by a wave which is guided by said optical fibers, especially for a bidirectional data-transmission system.
2. Description of the Prior Art
This type of optical component is employed extensively in optical-fiber data-transmission systexs. In particular, when bidirectional transmission takes place between two stations coupled by means of an optical-fiber channel link, it is necessary to transmit a data-carrying light wave from a first station to a second station via said channel link. The second station transmits return data by means of a wave which is guided by the safe channel link. It is then necessary to extract all or part of the light energy carried by said guided wave.
The energy extraction rate usually has to be adjusted in order to take into account many optoelectronic parameters associated on the one hand with the channel links between stations and on the other hand with the stations themselves.
Furthermore, when the configuration of the system is subject to modifications, readjustments have to be made in order to take these modifications into account.
Finally, in certain transmission systems, the links are not symmetrical. This is generally the case, for example, with the so-called teledistribution networks.
Teledistribution is already in widespread use in certain countries and is expected to develop at a higher rate over the next few years. This development is facilitated by the use of optical-fiber transmission cables.
In the most simple types of teledistribution networks, a central station transmits via a transmission cable connected to subscriber stations either one or a number of television programs, AM/FM radio broadcasting prograxs and even messages for private subscribers. Selection is carried out by making use of filters in order to tune the subscriber's receiver to a predetermined frequency band associated with a particular program. In the mcre elaborate networks, bidirectional transmission systexs are employed. In addition to the facilities mentioned in the foregoing, these networks permit the connection of equipment such as telex devices, for example, to the teledistribution system. The subscriber can accordingly transmit orders to the central station for selecting a particular program (TV, AM/FM) which will alone be transmitted to the subscriber, transmit orders relating to the other available services, or transmit his own signals.
Transmission of video signals calls for high bit-rate communication links or, equivalently, wide-band links. In the field of teledistribution, the links initially employed consisted of electric cables of the coaxial type, for example. In order to provide a higher bit rate, it is an advantage to replace these electrical links by optical-fiber links.
On the other hand, the signals transmitted by the subscriber usually occupy only a limited frequency-band for this type of application.
Furthermore, the emitting power of light sources is usually lower in subscriber stations than at the central station.
It is therefore necessary to provide couplers having an adjustable energy-extraction rate. It is further apparent from the foregoing that these couplers must provide a wide range of adjustment if it is desired for reasons of standardization and economy to employ the smallest possible number of separate components.
In the prior art, a number of different approaches have been proposed for the construction of an optical coupler which provides a variable light-energy extraction rate.
A first approach was described in U.S. Pat. No. 4,103,154. The device described in this specification comprises two bundles of optical fibers connected optically by means of a single optical fiber having a cross-sectional area substantially equal to the cross-sectional area of the fiber bundles, mechanical coupling being achieved by means of male and female connectors. The single fiber has a core and a cladding. A window is forxed in the cladding, the area of said window being determined as a function of the optical power to be derived. A conventional photodiode consisting of a base surmounted by a photodetector chip which determines the light-detecting surface area is provided opposite to said window. A drop of a bonding product is deposited on the detecting surface and the assembly is bonded to the single optical fiber at the level of the window formed in the fiber cladding.
The method of formation of said window consists in removal by abrasion of part of the optical fiber cladding material. By rubbing a cylinder against the cladding, a substantially elliptical opening is thus obtained. The portion of fiber core which has thus been bared is then polished. Thus a fraction of the radiation which arrives on the interface between the fiber core and the bonding material is transmitted by said material to the detecting face of the photodiode. As the opening formed in the cladding and occupied by the bonding material is of greater area, so the detected fraction of incident radiation is larger. These arrangements therefore permit adjustxent of the derived fraction of optical power at the time of fabrication but fail to permit an adjustment operation proper and especially an adjustment in situ.
A second approach was described in an article by Gfeller and Bapst which appeared in "Electronics Letters", Vol. 15, July 19th, 1979, pages 448-450.
It is known that, in the case of an emergent guided wave, the end face of an optical fiber behaves as a source of divergent light. By virtue of the symmetry of revolution of an optical fiber, the output beam is a conical beam having an axis of symmetry which coincides with the optic axis of the optical fiber and having a gaussian distribution in a radial direction.
If a second optical fiber having an optic axis which coincides with that of the first fiber is placed in oppositely-facing relation, depending on the distance between the end-faces of the two fibers, the angle of incidence of part of the light rays on the entrance face of the second fiber is larger than the ultimate value which permits guidance of the light wave within the core of the second fiber.
In the article cited above, it is proposed to place around the second optical fiber a waveguide tube for capturing the light rays which escape from the fiber core and for guiding them towards the exterior of a connector and finally, after reflection from a beveled external face of said tube, the light rays are detected by means of a photodetector located outside the connector.
Apart from the fact that the method adopted results in a fairly complex structure, it is not possible to vary the energy-extraction rate to a substantial extent and part of the energy emitted by the end-face of the first optical fiber is employed or in other words is neither guided by the second optical fiber nor extracted.